Post Tension Friction-Loss Calculations

The friction-loss calculations are important submittals for post-tensioning tendons. The calculations determine what the tendon’s final force is after short-term and long-term friction losses.

The key input parameters for “Long-Term Losses” are as follows:

  • Post-Tensioning System (Unbonded or Bonded)
  • Type of Strand (usually Low-Lax)
  • Ultimate Strength of Strand (usually 270 ksi)
  • Modulus of Elasticity of Strand (usually 28,500 ksi)
  • Estimate of initial average compression (depends on concrete member)
  • Concrete Strength at 28 days (varies, but typically 5000 psi)
  • Average Weight of Concrete (Normal or Light-weight)
  • Estimate Age of Concrete at Stressing (usually 3 days)
  • Modulus of Elasticity of Concrete at Stressing (ex. 57 x sqrt of specified 3-day compressive strength of concrete of 3000 psi = 3122 ksi)
  • Modulus of Elasticity of Concrete at 28 days (ex. 57 x sqrt of specified 28-day compressive strength of concrete of 5000 psi = 4030 ksi)
  • Estimate of Average Relative Humidity (varies by geographical region)
  • Volume to Surface Ratio of member (depends on concrete member)

The key input parameters for “Friction & Elongation” are as follows:

  • Coefficient of angular friction (varies by PT supplier)
  • Coefficient of wobble friction (varies by PT supplier)
  • Ultimate Strength of Strand (usually 270 ksi)
  • Ratio of Jacking Stress to Strand’s Ultimate Strength (0.80 per ACI-318)
  • Anchor Set (usually 0.25?)
  • Cross-Sectional Area of Strand (varies, but usually 0.153 sq.in. for 0.5? diameter strand)
  • Total Number of Strand per Tendon (1 for unbonded, varies for bonded)
  • Stressing Configuration (Single-End or Double-End)
  • Length and Drape of Tendon per Span (depends on design of concrete member)

The output will include the following:

  • Long-Term Losses
    • Elastic Shortening
    • Shrinkage
    • Creep
    • Relaxation
  • Final Average Force in Tendon
  • Total Elongation
  • Critical Stress Ratios

ACI-318 and PTI Manuals go into further detail about analyzing the results of the friction-loss calculations.

– Neel Khosa, Vice President, AMSYSCO

Related Articles:

Video on Field Friction Test

Material Properties of Post-Tension Strand
____________________________________________
Copyright © 2011 by AMSYSCO, Inc. All rights reserved.

Share
This entry was posted in Design Issues and tagged , , , , , , , . Bookmark the permalink.

Leave a Reply